1. Field of the Invention
The present invention relates to a termination for an electric cable, for medium and high voltages, of the type for outdoor use, exposed to air and possible polluting agents, complete in all its parts before being transported to installation place and suitable for connection to the electric cable without accessing into the inside thereof.
2. Description of the Related Art
For the purposes of the present invention, with medium and high voltages it is generally meant voltages in a range of from 10 kV to 245 kV and more.
For the purposes of the present invention, the term xe2x80x9ctermination for an electric cablexe2x80x9d means a device suitable to connect an electric cable to a bare conductor, such as, for example, an overhead conductor. This termination comprises electrical connecting means between the cable conductor and the bare conductor, and electrical separation means between the outer surface of the cable and parts connected thereto, generally at ground potential, and the elements under tension, such as the bare conductor. A termination of this kind generally comprises a conductive element, an insulating coating, and field control means able to produce an electric field configuration compatible with dielectric strength features of the relevant elements.
A known termination for outdoor use is described in the publication xe2x80x9cNew Prefabricated Accessories for 64-154 kV Crosslinked Polyethylene Cablesxe2x80x9d (Underground Transmission and Distribution Conference, 1974, pages 224-232).
The termination comprises a supporting base for an outer insulating casing made of porcelain and provided with a finned surface. The electric cable devoid of its shield extends over the whole length of the termination up to the upper extremity where it is fastened to be supported by the casing and where connection occurs to a voltage shank and therefrom to another electric installation.
A cylinder made of an epoxy resin incorporating a ground electrode is assembled about the elastomer element and is fastened to the supporting base. The cylinder has a conical shape matching that of the cone-shaped element inside it.
The installation further comprises the use of a spring system acting on the lower end of the elastomer element so that a pressure between such element and the cable insulation takes place and also between the element and the epoxy resin cylinder, so as to avoid the presence of air traces and thus the risk of electrical discharges.
The space inside the porcelain casing is filled with insulating oil.
The termination, due to the presence of a porcelain casing, is of the self-bearing type and rests on a base being part of a pylon or the like.
Also known are terminations provided with a solid insulator of the overhead type instead of resting on a supporting structure. According to this solution the termination is associated with the structure by interposition of an insulating element.
For example, from publication xe2x80x9cIEEE Power Cables and Accessories 10 kV-180 kVxe2x80x9d, London, November 1986, pages 238-241, it is known a 66 kV voltage termination for an electric cable which is extended over the whole length of the termination.
In this solution, starting from one cable end the semiconductive shield is removed over a certain length thereof and the shield end is coated with a conductive paint until an insulator portion is covered.
On the area where the shield is cut off, heat-shrinkable tubes of given impedance due to the presence of capacitive and resistive elements are applied, which tubes capacitively associated with the cable conductor reduce electrical stresses in the area where the shield is cut off.
An additional shrinkable tube is then disposed about the innermost tubes of same type and about the cable insulator portion.
The outermost tube comprises an insulating profile close to the cutting-off end of the cable shield.
A plurality of fins of heat-shrinkable insulating material is then fitted on the outer tube.
For installation of the termination qualified workers skilled in heat-shrinking technique are required and accuracy, attention to details and cleanliness in all steps are needed.
Patent WO 91/16564, in its part describing the state of the art, mentions terminations made, as the previous one, according to heat-shrinkage technique, and explains how to avoid that technique by making use of a suitable mandrel on which a jacket of elastomer material provided with finnings is expanded; the jacket is contracted by slipping off the mandrel onto a previously prepared electric cable of smaller diameter.
The final cable portion to be enclosed within the termination comprises the cable conductor, followed at a certain distance by the insulator devoid of a semiconductive layer and the shield.
A tubular winding is part of the cable assembling, made of a material formulated so as to reduce stresses otherwise present at the end of the semiconductive layer; this winding is applied on the semiconductive layer and the insulator.
Alternatively, provision may be made for a layer intended for reducing said stresses within the inner insulating jacket, by coextrusion or rolling for example.
The upper end of the conductor is associated with a flat clamp provided with a hole.
To make slipping the mandrel off easier, a plurality of ribs may be provided on which the elastomer jacket rests, and also the use of a lubricant between the ribs.
The mandrel is disposed with its elastomer jacket about the cable end portion already arranged to be part of the termination. The jacket twisting on the mandrel facilitates the lubricant spreading on the ribs and the mandrel slipping off, thereby causing contraction of the jacket on the cable end portion.
Also known are terminations for electric cables in which the electric cable is inserted only partially into the termination instead of extending up to the shank connecting to the overhead line or other electric installation.
One example of such self-bearing termination is described in patent application EP 95 101 33 82 of the Applicant itself.
In this solution, the electric cable is stopped at a given point from the termination entrance and the electric connection over the whole length which is required to reach the overhead line is accomplished by means of a conductive element.
The conductive element is externally provided with a finned elastomer layer and is supported by an insulating base preferably made of an epoxy resin resting on a supporting element.
The assembly consisting of the epoxy resin base and the conductive element makes the termination of the self-bearing type.
In particular, the assembly consisting of the epoxy resin base and the rigid conductive element brings the electric field to the termination surface to a value compatible with the dielectric strength of air and is long enough to form a leak line adapted to resist discharges.
The electric cable, at a given distance from the termination entry, is deprived of the shield, leaving the insulator uncovered. At the shield cutoff, electric field control means are provided which consist of a deflecting cone and an upper insulator useful for giving rise to electric field values compatible with the electrical strength of the employed materials and the surrounding air.
The epoxy resin base comprises a cone-shaped cavity on which the end portion of the insulator of the field control means rests.
A suitable spring arrangement pushes the cone and the insulator over it against the cone-shaped surface of the resin base in such a manner that no incorporation of air bubbles may occur between the contact surfaces of the different parts, which as known may bring about risks of electrical discharges.
WO 97/09762 discloses a cable termination for a high-voltage cable insulated by a solid insulating material which is used to connect the cable with an insulated conductor. With a particular arrangement of electrical couplings and field-control elements between the cable conductor and the insulated conductor, this device allows a rapid and leakage-free installation. This device forms a closed current path without the risk of flashover. This device is primarily (even though not exclusively) designed for connecting solid-insulated cables with gas-insulated devices. In particular, in FIG. 2 a connection between a solid-insulated cable and a gas-insulated insulator for outdoor use is illustrated.
The Applicant has noted that, by arranging a cable termination completed in all its parts before being mounted into its support and location of use, and by subsequently carrying out the operations of connection to the cable, installation operations can be substantially simplified and reliability of the operating assembly increased. Such connection to the cable operations employ connection, junction, or lapping techniques falling within the standard technical knowledge of the installation user, generally an electric energy supplying company or the like.
Within the scope of the present invention it has been found, in particular, that a termination having a portion substantially of the same size as the cable for which the termination is intended allows ready connection, by means of known techniques, of the termination to the cable. In this way, all assembly operations and technical tests on the termination could be carried out at the factory, in a controlled environment and by a qualified staff, without requiring performance of these operations in the field or by the customer.
In a first aspect, the invention relates to a termination assembly for an electric cable, comprising:
a termination having a conductive element and an insulating covering extended over it and
means for connection to an electric cable, characterized in that
said termination comprises a connecting portion comprising a conductor and an insulating covering coaxial thereto, having a substantially cylindrical configuration and predetermined lengths and diameters, and
said connecting means comprise a joining assembly which connects said connecting portion of the termination with one end of said electric cable,
wherein said predetermined lengths and diameters are compatible with said joining assembly.
In a preferred embodiment, said insulating covering of the termination comprises an insulating body having means for connection with an outer supporting structure.
In particular, said connecting portion of said termination comprises an end portion of said conductive element and said insulating body.
Preferably, said joining assembly connecting said connecting portion of the termination and one end of said electric cable comprises an electric connection element and a prefabricated flexible joint. The electric connection element joins a conductor of said electric cable and said conductive element of said termination. The prefabricated flexible joint covers said electric connection element and a portion of said connecting portion of the termination and of said end of said electric cable.
In another aspect, the present invention relates to a termination for an electric cable of predetermined diameter, comprising:
a conductive element longitudinally extended between one lower extremity and one upper extremity,
a conductive shank connected to said upper extremity of said conductive element, adapted to connect the termination to an electric installation,
an insulating body surrounding said conductive element,
a casing of insulating material about said insulating body,
field control means included in said casing, characterized in that said conductive element comprises a substantially cylindrical portion including said lower extremity, and said insulating body comprises a portion surrounding said lower extremity of the conductive element, having an outer cylindrical surface coaxial with said substantially cylindrical portion of said conductive element.
In particular, said insulating body comprises a conductive coating extended over a predetermined length of said portion surrounding the lower extremity of the conductive element, suitable to constitute an electric shield about the conductive element itself.
Preferably, said field control means are in contact with said conductive coating of the insulating body.
In a preferred embodiment said field control means comprise a field deflecting element of semiconductive elastomeric material.
Preferably, said conductive coating on the insulating body comprises a paint layer filled with conductive fillers.
Preferably, said insulating body comprises means for connection to an outer supporting structure.
In particular, said connecting means comprise a flange transverse to the longitudinal axis of the termination comprising a supporting surface for the terminal to the supporting structure.
Preferably, in an aspect of the present invention, said conductive element and said insulating body constitute a substantially rigid assembly, adapted to resist a predetermined transverse stress.
Preferably, said casing of insulating material about said insulating body comprises a portion of elastomeric material adhering to the insulating body or elastically tightened on the insulating body.
Preferably said casing of insulating material comprises an outer part provided with a finned surface; preferably said outer part with a finned surface consists of a material having environmental resistance.
In a preferred embodiment, said first part of the insulating assembly incorporates a deflecting element elastically pushed along the lower portion of the insulating body in contact with the conductive coating applied to the insulating body forming an electric shield.
In a particular embodiment, said insulating body incorporates a deflecting element in the lower part thereof and said insulating casing is formed of a finned tube elastically and directly fitted on the outer surface of the insulating body.
In a further aspect, the present invention relates to a method of connecting an electric cable, comprising an electric conductor, an insulating covering and a shield disposed about said conductor, to a bare conductor, characterized in that it comprises the steps of:
suitably arranging a termination comprising a conductive element, an insulating body about said conductive element, a casing of insulating material on said insulating body and a conductive shank connected to a first extremity of said conductive element;
arranging a conductive coating adapted to form an electric shield over a predetermined extent of the lower portion of the insulating body,
providing electric field control means at an extremity of said conductive coating of said insulating body,
and subsequently
carrying out a mechanical and electric connection between a second extremity of the conductive element of the termination and the cable conductor,
insulating said mechanical and electric connection,
connecting the cable shield to said conductive coating of the termination.
Preferably, the method of the invention further comprises the step of mechanically connecting said insulating body to a supporting structure.
Preferably, said steps of insulating said mechanical and electric connection and connecting the cable shield with said conductive coating of the termination comprise applying a joint over the area corresponding to said mechanical and electric connection between a second extremity of the conductive element of the termination and the cable conductor.
In a particular embodiment, said step of arranging a conductive coating adapted to form an electric shield over a predetermined extent of the lower portion of the insulating body comprises applying a semiconductive paint to the insulating body.
Preferably, said electric field control means have at least one portion pressed on the conductive coating of the insulating body.
In a preferred embodiment said casing is elastically fitted about said insulating body.
In particular, in accordance with the method of the invention, said steps of:
suitably arranging a termination comprising a conductive element, an insulating body about said conductive element, a casing of insulating material on said insulating body and a conductive shank connected to a first extremity of said conductive element;
arranging a conductive coating adapted to form an electric shield over a predetermined extent of the lower portion of the insulating body,
disposing electric field control means at an extremity of said conductive coating of said insulating body, are carried out at the factory before installation of the termination in the field.
Preferably, the method of the invention further comprises the step of electrically testing said termination carrying said conductive covering and electric field control means at the factory, before installation.
In accordance with a further aspect, the present invention relates to a method of manufacturing a termination for an electric cable characterized by the steps of:
arranging a conductive element linearly extended over a predetermined length,
applying an insulating body about the conductive element,
externally applying a conductive coating over a predetermined extent to a portion of the insulating body close to one of the extremities of the conductive element,
applying electric field control means in electrical contact with said conductive coating,
disposing a casing of insulating material having a predetermined surface extension over said insulating body and over said electric field control means, in an area corresponding to the extremity of said conductive element and said electric field control means.
Further features will become more apparent from the detailed description of some preferred embodiments of a termination for an electric cable in accordance with the present invention.